1. In a large bowl, beat the softened butter and granulated sugar together until light, fluffy, and pale in color, about 3 to 4 minutes.

2. Add the egg and egg yolk one at a time, mixing well after each addition and scraping down the sides of the bowl.

3. Beat in the lemon zest, lemon juice, and vanilla extract until the mixture is smooth and silky.

4. In a separate bowl, whisk together the flour, baking powder, and salt. Add the dry ingredients to the butter mixture in two additions, mixing on low just until a soft dough forms.

5. Divide the dough into two equal portions, shape each into a flat disk, wrap tightly in plastic wrap, and chill in the refrigerator for at least 1 hour or until firm enough to roll.

6. Preheat the oven to 350°F (175°C) and line two baking sheets with parchment paper.

7. Working with one dough disk at a time on a lightly floured surface, roll the dough to about 1/4 inch thickness. Cut out tree shapes with a Christmas tree cookie cutter and transfer them to the prepared baking sheets, spacing about 1 inch apart.

8. Gather and re-roll scraps once or twice to cut additional cookies, being careful not to overwork the dough.

9. Bake the cookies for 9 to 11 minutes, rotating the pans halfway through, until the edges look set and the bottoms are just barely golden while the tops remain pale.

10. Let the cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely before icing.

11. To make the lemon icing, whisk together the 3 cups powdered sugar, 3 tablespoons lemon juice, 3 tablespoons milk or cream, and the light corn syrup if using until smooth and pourable but not runny. Adjust with a few drops more milk or powdered sugar as needed.

12. Tint the icing with yellow gel food coloring a little at a time until you reach a bright lemony shade.

13. Dip the tops of each cooled cookie into the yellow icing or spoon it over the surface, letting excess drip back into the bowl, then place the cookies on a rack set over parchment paper and allow the icing to begin setting.

14. For the white drizzle, whisk together 1 cup powdered sugar, 2 1/2 tablespoons milk, and 1/2 teaspoon vanilla until smooth and just thin enough to drizzle. Transfer to a piping bag or small zip-top bag and snip a tiny corner.

15. Drizzle the white icing in zigzags over each yellow tree to mimic garlands, then immediately sprinkle with white sanding sugar or sparkling sugar so it adheres.

16. Let the cookies stand at room temperature until both icings are fully set and dry to the touch before stacking, serving, or packaging for gifting.